Neonates are more prone to fluid and electrolyte disorders than adults due, in part, to the immature kidney. We have shown that there are marked differences between the rate and characteristics of neonatal proximal tubule NaCl and NaHCO3 transport than that in the adult. Proximal tubule reabsorption of NaCl and NaHCO3 is dependent in large part on the luminal Na+/H+ exchanger, which in adults is NHE3. Neonates have almost no apical membrane NHE3 despite having a significant amount of Na+/H+ exchanger activity. We provide evidence that the neonate has a different Na+/H+ isoform than the adult. We show that the brush border membrane of the neonatal proximal tubule expresses abundant NHE8 and little NHE3, compared to the adult that primarily expresses NHE3 and little NHE8. However, we know very little about the functional characteristics NHE8, its regulation and what causes the isoform switch during postnatal development to NHE3. The aims of this proposal are to characterize the function of NHE8 in vitro and in vivo. We will use a highly sensitive technique of pH selective electrodes on the whole-cell patch configuration where we have total control of the extra and intracellular milieu to characterize the function of NHE8. We will examine the factors responsible for the isoform switch from NHE8 during postnatal development using a novel animal model to study renal development, the adrenalectomized-hypothyroid rat, where the maturation of proximal tubule Na+/H+ exchange and NHE3 is nearly arrested at the neonatal level. We have shown that NHE3 levels are increased by the glucocorticoids and thyroid hormone. The adrenalectomized-hypothyroid rat will allow us to test the hypothesis that the relative hypothyroid, glucocorticoid deficient state of the neonate prevents the expression of NHE3 making another isoform with different regulatory properties necessary for neonatal proximal tubule acidification. We will examine if NHE8 is regulated in the opposite direction by glucocorticoids and thyroid hormone resulting in down regulation in the adult. Finally, we now have a NHE8 null mouse that will allow us to examine the relative functional importance of this developmental NHE isoform in renal acidification and its relative importance in response to acidosis in both neonates and adults.